Our Approach to Pipeline Walking: Assessment and Mitigation
Overview
Pipeline walking is the gradual accumulation of axial displacement in a subsea pipeline subjected to repeated thermal and pressure cycles. This phenomenon typically occurs in high-pressure, high-temperature (HPHT) systems where the pipeline is not fully restrained axially. It can lead to excessive expansion and over-rotation at pipeline ends and in-line tees, or excessive lateral displacements at planned buckles.
These effects may collectively contribute to overstress of the pipeline, tie-ins, or other connection points.
"Pipeline walking may overstress pipeline, tie-ins, or other connection points."
Mechanisms of Pipeline Walking
Walking can be triggered by several independent or combined mechanisms:
- Pipeline slope: A sloping seabed induces a component of the pipelines submerged weight acting downslope. When axial resistance is mobilised cyclically during start-up and shutdown, this gravitational component causes incremental downhill movement that accumulates over time.
- Thermal expansion asymmetry: Differences in temperature change or end restraint stiffness between the two ends of the pipeline generate unbalanced axial forces. This asymmetry produces a net residual displacement after each thermal cycle, even when the pipeline is otherwise straight and level.
- Variations in internal fluid density: In multiphase flow systems, transient density gradients along the pipeline during flow rate changes or slugging create spatially varying internal pressures. The resulting axial force imbalance can drive small incremental movements that progressively accumulate with each transient.
"Slope, temperature transients, and fluid density variations are the main drivers of pipeline walking."
Mitigation Strategies
Pipeline walking is typically mitigated during the design phase by the inclusion of axial restraints or anchors along the pipeline route. These elements limit free axial movement.
For pipelines already in operation, standard or clamped concrete mattresses can be applied to stabilise the pipeline. They increase axial resistance, reducing displacement at tees and ends, and limiting lateral movement at planned buckles, helping maintain structural integrity without disrupting operations.
When assessing pipeline walking, we combine engineering judgement with bespoke three-dimensional Abaqus finite element models. Our approach begins by reviewing the key drivers of walking, including pipeline slope, thermal expansion patterns, transient multiphase fluid conditions, pipe-soil interaction, tie-in configuration, and the presence of lateral buckles.
Assessment and Approach
The models are used to evaluate cumulative axial displacement over repeated operating cycles, focusing on expansion at tie-ins and in-line tees. The analysis also accounts for pipe-soil interaction, including both axial drained and undrained soil conditions, to capture the effect of soil behaviour on axial displacement and overall pipeline response.
In addition, the analysis considers planned buckle mitigation, the potential influence of rogue buckles, and berms at rogue buckles, which alter the residual force at the buckle and consequently affect the overall force profile along the pipeline length. The models also simulate the evolution of the buckle shape for both rogue and planned buckles, providing insight into localised deformation and the distribution of residual forces along the pipeline.
Sensitivity analyses are performed to identify the parameters that most significantly influence pipeline walking behaviour. Thanks to the robustness and efficiency of our models and the use of Python coding for batch pre- and post-processing, we can run simulations accounting for the impact of full design cycles across multiple sensitivities.
This methodology provides a comprehensive and adaptable framework for quantifying displacement, assessing mitigation needs, and managing pipeline walking.